Recently, to maximize performance and communication capability of wireless communication systems, attention is paid to a multiple input multiple output (MIMO) system. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas in order to improve efficiency of data transmission and reception. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then assembled into one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can increase with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit
diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight factor to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. The weight factor can be represented by a weight matrix which is referred to as a precoding matrix. The precoding matrix may be directly transmitted between a base station (BS) and a user. Alternatively, a precoding matrix index (PMI) may be transmitted instead of directly transmitting the precoding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as a spatial division multiple access (SDMA) or a multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt, and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Nr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed. The precoding matrix may be a precoding vector when a rank is 1.
In the MU-MIMO system, pairing can be performed on radio resources for multiple users. Pairing denotes allocation of radio resources to the multiple users by entirely or partially overlapping the radio resources over a frequency domain or a time domain. The multiple users can share the overlapping radio resources over the frequency domain or the time domain. In the MU-MIMO system, pairing can be performed on data of users, wherein the number of users corresponds to the number of Tx antennas. For example, if a BS has 4 Tx antennas, the BS can perform pairing for 2 to 4 users.
In a region where the radio resources are paired, data of other users acts as interference to data of a current user. Thus, there is a need for a method for effectively canceling interference caused by data of other users from data of a current user.